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The Mystery Fault, Part 3

In Part 2, we’ve established that the Silver Creek fault is, in fact, the southwestern boundary of the Evergreen basin. But what’s it doing there, anyhow? Back sometime around 10 to 15 million years ago, an early Hayward fault formed (an estimated 100 km (62 miles) south of where it is now) with a right stepover in the southeastern most end of it. Basically that’s where a fault stops and then starts again some distance away, in this case to the right. .Now, both the Hayward and the Silver Creek faults are right-lateral strike-slip faults. This means if you stand facing the fault, and it slips, the ground on the other side of the fault will move right. When you have a right step in a right-lateral fault system, it tends to pull the ground between the faults apart like so:

And thus creating a pull-apart basin. So the basin started to be created in the Miocene. Since then there’s been 175 km (109 miles) of slip on the Hayward Fault, of which 40 km (25 miles) of slip was on the Silver Creek fault itself.

Now, the Hayward fault is a very active fault. Parts of it are creeping at the surface, but the whole fault system is locked at depth right now and is one of the prime candidates for the next Northern California Big One. Scary thought, and keeps geologists and emergency managers awake at night. But what about the Silver Creek fault?

Well, it turns out that about 2 million years ago, give or take a few thousand, the slip that was happening on the Silver Creek fault moved over to the Calaveras fault, which runs east of the Hayward fault. (See the faults pic in Part 1.) That doesn’t mean the fault isn’t still moving, but it’s moving at a much slower rate than the Calaveras and Hayward faults, and probably won’t generate any big earthquakes. Probably.

There’s some evidence for a couple of magnitude 6-ish quakes in 1903 down near the south end of the Silver Creek fault, but they’re not very well located, and it could very well be that they were produced by the Calaveras or another fault. Because there is another fault, one that I haven’t talked at all about yet, and which is completely buried and only inferred. But there’s some pretty decent inference for inferring it. I’ll talk about it in Part 4.

Meanwhile, those of you who live in California, there are fault systems in both the northern and southern parts of the state primed to go off and produce the Next Big One. Plan for a few days of food, water, no electricity, etc. Expect cell phone systems to be down. Expect land lines to be overloaded. Expect hospitals to be backed up. But, as Douglas Adams says, don’t panic.